An electron spin resonance probe (5-doxyl methyl stearate) has been used to characterize the fluidity of lung Type II cell lamellar bodies which store pulmonary surfactant. These lecithin containing multilaminate structures were considerably less rigid than erythrocyte ghosts or normal leukocyte membranes, and were not changed by freezing or sonication. The lamellar bodies showed a temperature-dependent fluidity profile which was identical to liposomes made from lamellar body-extracted lipids; however, it differed significantly from lipasomes made of pure dipalmitoyl lecithin (DPL). Furthermore, liposomes made from a DPL-phosphatidyl glycerol combination which closely resembled the major lamellar body phospholipid components, were significantly different from the natural lamelar body liposomes. This indicates that perhaps minor phospholipids may play an important role in determining the molecular order maintained within the lamellar bodies.